Pharmacological properties : תכונות פרמקולוגיות

Pharmacodynamic Properties

5.1       Pharmacodynamic properties
Pharmacotherapeutic group: Anti-vertigo preparations. ATC-Code: N07CA01 
The mechanism of action of betahistine is only partly understood. There are several plausible hypotheses that are supported by animal studies and human data:
• Betahistine affects the histaminergic system:

Betahistine acts both as a partial histamine H1 -receptor agonist and histamine H3- receptor antagonist also in neuronal tissueand and has negligible H2-receptor activity. Betahistine increases histamine turnover and release by blocking presynaptic H3-receptors and inducing H3-receptor downregulation.

• Betahistine may increase blood flow to the cochlear region as well as to the whole brain: 
Pharmacological testing in animals has shown that the blood circulation in the striae vascularis of the inner ear improves, probably by means of a relaxation of the precapillary sphincters of the microcirculation of the inner ear. Betahistine was also shown to increase cerebral blood flow in humans.


• Betahistine facilitates vestibular compensation:

Betahistine accelerates the vestibular recovery after unilateral neurectomy in animals, by promoting and facilitating central vestibular compensation;this effect characterized by an up-regulation of histamine turnover and release is mediated via the H3 Receptor antagonism.. In human subjects, recovery time after vestibular neurectomy was also reduced when treated with betahistine.

• Betahistine alters neuronal firing in the vestibular nuclei:

Betahistine was also found to have a dose dependent inhibitory effect on neuronal activation in lateral and medial vestibular nuclei.

The pharmacodynamic properties as demonstrated in animals may contribute to the therapeutic benefit of betahistine in the vestibular system.

The efficacy of betahistine was shown in studies in patients with vestibular vertigo and with Ménière’s disease as was demonstrated by improvements in severity and frequency of vertigo attacks.

Pharmacokinetic Properties

5.2 Pharmacokinetic properties

Absorption
Orally administered betahistine is readily and almost completely absorbed from all parts of the gastro-intestinal tract. After absorption, the drug is rapidly and almost completely metabolized into 2- pyridylacetic acid. Plasma levels of betahistine are very low. Pharmacokinetic analyses are therefore based on 2- PAA measurements in plasma and urine.
Under fed conditions Cmax is lower compared to fasted conditions. However, total absorption of betahistine is similar under both conditions, indicating that food intake only slows down the absorption of betahistine.

Distribution:

The percentage of betahistine that is bound by blood plasma proteins is less than 5 %.
Biotransformation:

After absorption, betahistine is rapidly and almost completely metabolized into 2- PAA (which has no pharmacological activity).
After oral administration of betahistine the plasma (and urinary) concentration of 2- PAA reaches its maximum 1 hour after intake and declines with a half-life of about 3.5 hours.
Excretion:

2 PPA is readily excreted in the urine. In the dose range between 8 and 48 mg, about 85% of the original dose is recovered in the urine. Renal or faecal excretion of betahistine itself is of minor importance.

Linearity

Recovery rates are constant over the oral dose range of 8–48 mg indicating that the pharmacokinetics of betahistine are linear, and suggesting that the involved metabolic pathway is not saturated.